The present invention relates to a binary image processing method and apparatus for outputting document image data inputted from an image input device through a binary image output device, and particularily to an image processing method and apparatus with mixture of simple binarization and pseudo intermediate tone for entirely obtaining an image of high quality by processing the letter portion of image data with a simple binarization image processing method and by processing the photograph portion of image data with pseudo intermediate tone processing method in processing an document image mixed with a letter and a phtograph.
Image data inputted from an image input device such as an image scanner, a facsimile, a digital copy press, or the like should be transformed and processed into several data formats suited to the condition of the requested object and a peripheral equipment.
In particular, the inputted image data should be binarized in order to output through a binary image output device such as a laser beam printer, a dot matrix printer, or the like.
As aforesaid, there have been conventionally used two as a method for binarizing image data as follows.
The one is a simple binarization processing method which discriminates whether the value of image data read is larger than or smaller than the fixed reference value, thereby simply processing as binary image data of black level or white level ("0" or "1" level).
The other is a psuedo intermediate tone processing method which artifically displays graduation of an original image by controlling the density of a binary pixel in order to reproduce the shading or graduation component of an original image.
The latter is called as a dithering processing method or a half-tone processing method.
Generally, in processing a document image by a simple binarization image processing method, an image of which contrast is extemely clear such as a letter and a line is reproduced as that of better quality. But an image having a graduation component such as a photograph, thus an image of which contrast is not clear is reproduced as that of poorest quality with the exception of the contour (the edge) portion of picture in which graduation is greatly changed.
On the other hand, the phenomenon occurred in processing a document image by a psuedo intermediate tone image processing method is opposed to that in aforesaid simple binarization image processing method.
In other words, in processing a document image by a psuedo intermediate tone image processing method, an image having a graduation component such as a photograph is reproduced as that of better quality. However, an image of which contrast is extemely clear such as a letter or a line is reproduced as that of poorer quality because of discontinuity of a stroke component.
In particular, when an image having a spatial frequency component is reproduced by said image processing method, the Moire phenomenon that the pattern is generated in the reproduced image is accompanied because of interference beat, so that the picture quality becomes very deteriorated.
As above roughly mentioned, because the prior image input device exclusively adapts either a simple binarization image processing method or a psuedo intermediate tone image processing method, thus because only one of two image processing methods is adapted, the picture quality of an image is entirely deteriorated in binary processing general document image mixed a letter and a photograph.
The techniques for half-tone processing image data is disclosed in more detail in U.S. Pat. Nos. 4,920,501 and 4,958,238.